#parent {
  height: 300px;
  width: 300px;
  margin-left: 50px;
  padding: 10px;
  border: 1px solid black;
}

#my {
  height: 200px;
  width: 200px;
  margin-left: 40px;
  margin-top: 20px;
  padding: 10px;
  border: 1px solid red
}

#child {
  display: block;
  height: 100px;
  width: 100px;
  margin-left: 40px;
  margin-top: 20px;
  padding: 10px;
  border: 1px solid black
}